1. Field of the Invention
This invention relates to a process for forming a ferroelectric film containing a perovskite type oxide, which is of a PZT type. This invention also relates to a ferroelectric film obtainable by the process for forming a ferroelectric film. This invention further relates to a ferroelectric device, which comprises the ferroelectric film, and a liquid discharge apparatus utilizing the ferroelectric device.
2. Description of the Related Art
Piezoelectric devices provided with a piezoelectric body, which has piezoelectric characteristics such that the piezoelectric body expands and contracts in accordance with an increase and a decrease in electric field applied across the piezoelectric body, and electrodes for applying the electric field across the piezoelectric body have heretofore been used for use applications, such as actuators to he loaded on ink jet type recording heads. As piezoelectric body materials, there have heretofore been widely used perovskite type oxides, such as lead zirconate titanate (PZT). The materials described above are ferroelectric substances, which have spontaneous polarization characteristics at the time free from electric field application.
It has been known from the 1960s that PZT having been doped with donor ions having a valence number higher than the valence number of substitutable ions exhibit the characteristics, such as ferroelectric performance, having been enhanced over the characteristics of genuine PZT. As the donor ions capable of substituting Pb2− ions at an A site, there have been known Bi3+ ions and various kinds of lanthanoid cations, such as La3+ ions. As the donor ions capable of substituting Zr4+ ions and/or Ti4+ ions at a B site, there have been known V5+ ions, Nb5+ ions, Ta5+ ions, Sb5+ ions, Mo6+ ions, W6+ ions, and the like.
The ferroelectric substances have heretofore been produced with, for example, a technique wherein multiple kinds of oxide particles containing constituent elements for a desired composition are mixed together, and wherein the thus obtained mixed particles are subjected to molding processing and firing processing, or a technique wherein multiple kinds of oxide particles containing constituent elements for a desired composition are dispersed in an organic binder, and wherein the thus obtained dispersion is coated on a substrate and fired. With the aforesaid techniques for producing the ferroelectric substances, the ferroelectric substances have been produced via the firing processing at a temperature of at least 600° C., ordinarily at a temperature of at least 1,000° C. With the aforesaid techniques for producing the ferroelectric substances, since the production processing is performed in the high-temperature thermal-equilibrium state, a dopant having a valence number, which does not match, is not capable of being doped at a high concentration.
Studies on doping of various kinds of donor ions to a PZT bulk ceramic material are described in, for example, “Effects of Impurity Doping in Lead Zirconate-Titanate Ceramics”, S. Takahashi, Ferroelectrics, Vol. 41, pp. 143-156, 1982. FIG. 10 is a graph illustrating FIG. 14 of the literature described above. Specifically, FIG. 10 is a graph showing a relationship between a donor ion doping concentration and a dielectric constant. In FIG. 10, it is illustrated that the characteristics become best at a donor ion doping concentration of approximately 1.0 mol % (corresponding to approximately 0.5 wt % in the cases of FIG. 10), and that the characteristics become bad in cases where the donor ion doping concentration is higher than approximately 1.0 mol %. It is presumed that, in cases where the donor ion doping concentration is higher than approximately 1.0 mol %, a part of the donor ions, which part are not capable of forming a solid solution because of valence number mismatch, undergo segregation at particle boundaries, and the like, and the characteristics are thus caused to become bad.
Recently, ferroelectric substances, in which the donor ions have been doped at the A site at doping concentrations higher than those in “Effects of Impurity Doping in Lead Zirconate-Titanate Ceramics”, S. Takahashi, Ferroelectrics, Vol. 41, pp. 143-156, 1982, have been disclosed in, for example, Japanese Unexamined Patent Publication Nos. 2006-096647, 2001-206769, 2001-253774, and 2006-188414.
A PZT type ferroelectric film, in which Bi has been doped at the A site at a doping concentration falling within the range of more than 0 mol % to less than 100 mol %, and in which Nb or Ta has been doped at the B site at a doping concentration falling within the range of 5 mol % to 40 mol %, is disclosed in Japanese Unexamined Patent Publication No. 2006-096647 (Claim 1 thereof). The disclosed ferroelectric film is formed with a sol-gel technique. The sol-gel technique is the thermal-equilibrium processing. With the ferroelectric film disclosed in Japanese Unexamined Patent Publication No. 2006-096647, such that the sintering may be promoted and such that the thermal-equilibrium state may be obtained, it is essential to dope Si as a sintering auxiliary. (Reference may be made to, for example, a paragraph [0108] of Japanese Unexamined Patent Publication No. 2006-096647.)
In Japanese Unexamined Patent Publication No. 2006-096647, it is described that, with the Bi doping at the A site, oxygen deficiency is capable of being suppressed, and electric current leakage is capable of being suppressed. (Reference may be made to, for example, a paragraph [0040] of Japanese Unexamined Patent Publication No. 2006-096647.) Also, in Japanese Unexamined Patent Publication No. 2006-096647, it is described that, as the doping concentration of Bi and the doping concentration of Nb or Ta are set to be high, rectangularity of polarization-electric field hysteresis is capable of being enhanced, and the polarization-electric field hysteresis is capable of becoming appropriate. (Reference may be made to, for example, a paragraph [0114] of Japanese Unexamined Patent Publication No. 2006-096647.)
A PZT type bulk sintered body, which contains 0.01% by weight to 10% by weight of Bi2O3 and 0.01% by weight to 10% by weight of GeO2, is disclosed in Japanese Unexamined Patent Publication No. 2001-206769. Also, a PZT type bulk sintered body, which contains 0.01% by weight to 10% by weight of Bi2O3 and 0.01% by weight to 10% by weight of V2O5, is disclosed in Japanese Unexamined Patent Publication No. 2001-253774. In Japanese Unexamined Patent Publication Nos. 2001-206769 and 2001-253774, it is described that, by the doping of Ge or V as the sintering auxiliary, the sintering processing is capable of being performed at a comparatively low temperature.
Further, a PZT type bulk sintered body wherein, for the valence number matching, Bi acting as donor ions having a high valence number and Sc or In acting as acceptor ions having a low valence number have been co-doped, is disclosed in Japanese Unexamined Patent Publication No. 2006-188414.
With the ferroelectric substances described in Japanese Unexamined Patent Publication Nos. 2006-096647, 2001-206769, and 2001-253774, such that the sintering may be promoted and such that the thermal-equilibrium state may be obtained, it is essential to dope Si, Ge, or V as the sintering auxiliary. However, in cases where the sintering auxiliary described above is doped, the ferroelectric characteristics become bad. Therefore, with the techniques described in Japanese Unexamined Patent Publication Nos. 2006-096647, 2001-206769, and 2001-253774, the effect of the donor ion doping at the A site is not always capable of being derived sufficiently.
Also, V used in Japanese Unexamined Patent Publication No. 2001-253774 acts as the donor ions at the B site. An ionic radius of V is smaller than the ionic radius of each of Nb and Ta, and it is considered that the effect of V as the donor ions will be smaller than the effect of each of Nb and Ta. Further, it is preferable that V2O5, which has a high toxicity, is not used.
With the ferroelectric substance described in Japanese Unexamined Patent Publication No. 2006-188414, for the valence number matching, the donor ions having a high valence number and the acceptor ions having a low valence number are co-doped. However, it has been known that the acceptor ions having a low valence number act to lower the ferroelectric characteristics. With the system, in which the acceptor ions are co-doped, the effect of the donor ion doping is not always capable of being derived sufficiently.
Also, with size reduction and weight reduction made in electronic equipment and enhancement of functions made in electronic equipment in recent years, there has arisen a tendency toward the reduction in size and weight of piezoelectric devices and enhancement of functions of the piezoelectric devices. For example, in the cases of the piezoelectric devices for use in the ink jet type recording heads, such that images having good image quality may be obtained, it has recently been studied to enhance array density of the piezoelectric devices. Further, such that the array density of the piezoelectric devices may be enhanced, it has recently been studied to reduce the thicknesses of the piezoelectric devices. The ferroelectric substances should preferably take on the form of a thin film.
In Japanese Unexamined Patent Publication Nos. 2001-206769, 2001-253774, and 2006-188414, the bulk sintered body is taken as the object. In Japanese Unexamined Patent Publication No. 2006-096647, the formation of the ferroelectric film with the sol-gel technique is described. With the sol-gel technique, in cases where the film thickness is set to be large, cracks arise readily. Therefore, with the sol-gel technique, it is not always possible to form a thin film having a thickness larger than 1 μm. In the use applications for ferroelectric memories, and the like, the ferroelectric film may be a thin film having a thickness of at most 1 μm. However, in the use applications for the piezoelectric devices, with the ferroelectric film having a thickness of at most 1 μm, sufficient displacement is not capable of being obtained. Therefore, in the use applications for the piezoelectric devices, the film thickness of the ferroelectric film should preferably be at least 3 μm. It will be possible to set the film thickness to be large with a technique, wherein the lamination of a thin film is iterated. However, the technique for iterating the lamination of a thin film will not be practicable. Also, with the sol-gel technique, Pb deficiency is apt to occur. In cases where the Pb deficiency occurs, there is a tendency for the ferroelectric performance to become bad.